First report of Paramyrothecium roridum causing Myrothecium leaf spot on soybean in Africa

Authors: Haudenshield, James; PAWLOWSKI, MICHELLE - University Of Illinois; MIRANDA, CARRIE - University Of Missouri; Hartman, Glen

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2018
Publication Date: 8/24/2018
Citation: Haudenshield, J.S., Pawlowski, M.L., Miranda, C., Hartman, G.L. 2018. First report of Paramyrothecium roridum causing Myrothecium leaf spot on soybean in Africa. Plant Disease. https://doi.org/10.1094/PDIS-04-18-0624-PDN.
DOI: https://doi.org/10.1094/PDIS-04-18-0624-PDN

Interpretive Summary: Paramyrothecium roridum causes Myrothecium leaf spot and has been reported as a pathogen on a wide range of plants, including soybeans, in the United States. To our knowledge, this fungus has not been reported on soybeans grown in Africa. In November 2016, collections of leaves from three different fields in Northern Ghana with leaf spot symptoms were sent by APHIS permit to the USDA-ARS Soybean Disease and Pest Laboratory for further verification. Tissue samples were plated on potato dextrose agar (PDA) and incubated at 25 C in the dark for 10 days. The isolates were white with dark green to black structures. Morphologically, the fungus appeared to be P. roridum. Pathogenicity was completed by inoculating detached leaflets of soybean cultivar Williams 82. Symptoms appeared at 3 days post inoculation as necrotic spots, a dark border with a chlorotic halo and dark green clusters of spores forming inside the necrotic lesion. The pathogen was re-isolated and cultured on PDA and appeared morphologically to be the same fungus. To further confirm the pathogen was P. roridum, DNA subsamples were sequenced to further confirm identity. This is the first confirmed report of P. roridum causing leaf spot on soybean in Ghana. The distribution of the pathogen on soybean in the U.S. is unknown. This report is important to plant pathologist and others interested in the distribution of plant diseases.

Technical Abstract: Myrothecium leaf spot, caused by Paramyrothecium roridum (Tode) L. Lombard & Crous (Basionym Myrothecium roridum) has been reported on a wide range of plants (Farr and Rossman 2018) including soybeans in the United States (Miller and Roy 1982). To our knowledge, this fungus has not been reported on soybeans grown in Africa. In November 2016, collections of leaves from three different fields in Northern Ghana with leaf spots symptoms were sent by APHIS permit to the USDA-ARS Soybean Disease and Pest Laboratory for further verification. Brown leaf spots ranging from 0.3 to 1 cm in diameter with dark edges and somewhat lighter centers often with scattered dark stoma, each 40 to 70 µm wide (n = 50) were observed on collected samples. Tissue samples were plated on potato dextrose agar (PDA) and incubated at 25 C in the dark for 10 days. The isolates were white with dark green to black stroma. The conidia developed from sporodochial conidiomata and were aseptate, hyaline and 5-(7.6)-10 µm x 1-(2.2)-3 µm (n = 50). Morphologically, the fungus appeared to be P. roridum. Pathogenicity was completed by inoculating detached leaflets of soybean cultivar Williams 82. Detached leaves were incubated inside clamshell containers and incubated with 12 hr light/dark cycles for 7 days. Symptoms appeared at 3 days post inoculation as necrotic spots a dark border with a chlorotic halo and dark green clusters of conidiomata inside the necrotic lesion. The pathogen was re-isolated and cultured on PDA and appeared morphologically to be the same fungus. To further confirm the pathogen was P. roridum, DNA subsamples were subjected to PCR using LSU and ITS primers and sequenced. The two top scoring BLAST hits for the LSU region had 96% coverage and 98% identity to P. roridum (M. roridum) and for the ITS1, the three top BLAST hits had 100% coverage and 99% identity for P. roridum, Myrothecium sp., and P. roridum (M. roridum). This is the first confirmed report of P. roridum causing leaf spot on soybean in Ghana. The distribution of the pathogen on soybean in the U.S. is not known.